1. Field of the Invention
The present invention relates to a power source arrangement for electric discharge machining by applying a voltage across an electrode and a workpiece.
2. Description of the Prior Art
When a workpiece is machined by applying a pulse voltage to a spark gap defined between an electrode and the workpiece, it is very likely that, during the machining, an arc will form in the spark gap or that cutting chips will accumulate in the gap to cause a short between the electrode and the workpiece. The generation of such an arc or short decreases the cutting efficiency in proportion to the duration of its existence and is liable to deteriorate the surface finish of the work. Especially, arcing or shorting of long duration markedly impairs the cutting efficiency and causes burning of the workpiece and, in the case of wire-cut, electric discharge machining, breakage of the wire. Accordingly, it is necessary to remove such arcing or shorting as soon as possible if it occurs during cutting.
To this end, the prior art employs a method wherein the OFF period of a power source DC for applying a voltage across an electrode D and a workpiece W, as shown in FIG. 1A, is extended for a certain period of time in the case of arcing or shorting. That is, during normal operation a switching element Q for turning the power source DC on and off is driven by a pulse signal of a predetermined period which has an ON time .tau..sub.on and an OFF time .tau..sub.off, such as shown in FIG. 1B, but in the case of arcing or shorting having once occurred, the switching element Q is driven by such a pulse signal as depicted in FIG. 1C which has long OFF and ON intervals Toff and Ton, thereby to quickly restore the insulation of the spark gap.
With the above conventional method, however, the ON and OFF intervals Ton and Toff are set at mean values obtained experimentally; therefore, an arc of high intensity cannot be removed in one OFF interval Toff. If the arcing is not removed in one OFF interval, then an arc discharge is produced in the immediately subsequent ON interval Ton in many cases, which degrades the insulation of the spark gap and makes it difficult to completely restore the insulation of the gap in the subsequent OFF interval Toff. For such reasons, the prior art has encountered difficulty in early removal of a high intensity arc and has suffered from burning of the workpiece and reduction of the cutting efficiency by arcing of long duration.